FIELD OF THE INVENTION
This invention relates to ballast circuits used for powering fluorescent tube lamps and in particular concerns a universal type ballast characterized by its simplicity and very low parts count, yet capable of supplying a wide range of loads while maintaining close regulation over the load current.
Fluorescent lamp ballasts may be generally categorized as magnetic ballasts or electronic ballasts. Magnetic ballasts have relatively few components as compared to the more complex electronic ballasts. The latter, however, are capable of very close load current regulation over a wide range of lamp load wattage. Fluorescent lamp tubes are available with various wattage ratings, most commonly from 8 W to 40 W, although higher and lower power lamps are available. A particular lighting installation may call for one or more lamp tubes to be supplied by a single ballast.
It is desirable to have a ballast which is capable of supplying the full range of commonly encountered lamp loads. While relatively simple magnetic ballasts can supply current to a range of lamp loads, the simpler ballast circuits generally provide inadequate regulation of the load current, which results in noticeable and unacceptable variations in light output from the lamp tubes and may also impair the life of the lamp tube. At present, applications requiring close regulation of load current for a wide range of lamp loads tend to use electronic ballasts. Past efforts to design small, lightweight magnetic ballasts of relatively simple configuration and capable of maintaining close current regulation over a wide load range have failed to produce devices which approach the regulation capability of electronic ballasts.
Simple magnetic ballasts have been known for a long time. In its simplest form, a magnetic ballast may consist of a single inductor connected in series with a fluorescent lamp tube. As the current through the lamp increases after striking of the lamp, the impedance of the inductor limits the current through the lamp tube to a safe operating value. Good regulation can be achieved with such an arrangement only with a large and therefore bulky and heavy ballast inductor, and using a high supply voltage, neither of which are practical in most applications. For reasonably sized inductors and commonly available supply voltages the value of the ballast inductor must be adjusted for each particular lamp load.
U.S. Pat. No. 2,301,891 issued to Lecorguillier discloses an inductance/capacitance bridge circuit of a so called constant voltage/constant current type for supplying power to a fluorescent lamp tube. Lecorguillier is concerned with stabilizing lamp operation in order to extend the life of the lamp electrodes under conditions of constant input voltage to the ballast circuit. The Lecorguillier patent, however, completely fails to address the problem of supplying a steady operating current to different lamp loads, and in particular does not recognize any ability of the inductance/capacitance bridge to closely regulate lamp current over a load wattage range of at least three to one. Further, Lecorguillier was unable to achieve stable lamp operation using only the inductance/capacitance bridge circuit. In an effort to overcome this deficiency, Lecorguiller makes use of an additional inductor connected in series with the lamp tube. This additional inductor is undesirable as it adds substantial weight and bulk to the ballast. A simpler solution is needed to achieve stable and dependable operation of the inductance/capacitance bridge circuit without resort to the series inductor. Still further, Lecorguiller neglects to provide an effective filament power supply for the lamp tube which reduces filament power after striking of the lamp tube in order to extend filament life and increase system efficiency.
A continuing need exists for a compact, lightweight magnetic ballast of simple design with few components which is capable of performance approaching or equal to that of electronic ballasts. This need is particularly felt in connection with aircraft passenger cabin lighting systems.